Haiss Peter, Zeller Klaus-Peter
Institut für Organische Chemie, Auf der Morgenstelle 18, 72076 Tübingen, Germany.
Org Biomol Chem. 2003 Jul 21;1(14):2556-8. doi: 10.1039/b304002j.
Ethyl 2-diazo-4,4,4-trifluoroacetoacetate (1a) and 3-diazo-1,1,1-trifluoro-2-oxopropane (1b) exhibit a deviating behavior in solution photolysis (hydrogen abstraction for 1a; Wolff rearrangement for 1b) [(a) F. Weygand, W. Schwenke and H. J. Bestmann, Angew. Chem., 1958, 70, 506; (b) F. Weygand, H. Dworschak, K. Koch and S. Konstas, Angew. Chem., 1961, 73, 409]. As shown by 13C-labelling of 1b this difference is not caused by rearrangement of the primarily formed alpha-oxocarbene to an isomeric alpha-oxocarbene presenting a hydrogen atom as a migrating substituent for the Wolff rearrangement. It is discussed that the singlet alpha-oxocarbene generated from 1a rapidly undergoes spin equilibration followed by hydrogen abstraction of the triplet alpha-oxocarbene. In contrast, due to a larger singlet-triplet splitting in the singlet alpha-oxocarbene generated from 1b, the intramolecular Wolff rearrangement on the singlet surface can efficiently compete with the singlet-triplet interconversion.
2-重氮基-4,4,4-三氟乙酰乙酸乙酯（1a）和3-重氮基-1,1,1-三氟-2-氧代丙烷（1b）在溶液光解中表现出异常行为（1a发生氢提取；1b发生沃尔夫重排）[（a）F. 魏根德、W. 施温克和H. J. 贝斯特曼，《德国应用化学》，1958年，70卷，506页；（b）F. 魏根德、H. 德沃夏克、K. 科赫和S. 康斯塔斯，《德国应用化学》，1961年，73卷，409页]。如对1b进行的¹³C标记所示，这种差异并非由最初形成的α-氧代卡宾重排为呈现氢原子作为沃尔夫重排迁移取代基的异构α-氧代卡宾所致。据讨论，由1a生成的单线态α-氧代卡宾迅速经历自旋平衡，随后三线态α-氧代卡宾发生氢提取。相比之下，由于由1b生成的单线态α-氧代卡宾中单线态-三线态分裂较大，单线态表面上的分子内沃尔夫重排可有效地与单线态-三线态相互转化竞争。
